U.S. Pat. Nos. 3,260,597 and 3,265,498 disclose controlling the density of a Dmin range referred to as "fog" by using a development inhibitor or a development inhibitor precursor in a color diffusion transfer photographic element.
However, the compounds illustrated in these patents inhibit not only unnecessary development but also the necessary development, thereby reducing image quality. Furthermore, temperature greatly influences the photographic development reaction. That is, the development proceeds slowly at low temperature and proceeds quickly at high temperature. Accordingly, excessive development is, particularly, liable to occur at high temperature. Therefore, it has been desired to discover a development inhibitor precursor which inhibits the occurrence of unnecessary development at about room temperature, controls the development so that the necessary development is not disturbed, and expands the allowable range of the processing temperature by controlling the occurrence of an excessive development at high temperature.
U.S. Pat. No. 4,009,029 (Hammond et al) discloses a development inhibitor precursor for this purpose.
On the other hand, various photographic additives used in photographic elements or the reaction products thereof must not have undesirable effects such as reducing the image quality of the photographic elements before and after processing the photographic elements. The compounds, such as 5-(2-cyanoethylthio)-1-phenyltetrazole, as described in the foregoing U.S. Pat. No. 4,009,029, form harmful compounds, which promote the collapse of certain kinds of image-forming dyes, by processing photographic elements containing them. When the additives forming such harmful compounds exist in photographic elements, the density of color images is reduced with the passage of time after processing which reduces image quality.
When these additives are incorporated in a cover sheet of a photographic unit, they must not reduce the pH controlling function of the cover sheet. When the additives described in, for example, Japanese Patent Publication (Unexamined) No. 138745/80 are incorporated in a cover sheet or the cover sheet containing these additives is preserved for a long period of time, the reducing function of pH is greatly impaired, whereby the pH in the system is kept high for an extraordinarily long period of time. This phenomenon sometimes gives bad influences such as the reduction of sharpness of images on the photographic element.
Accordingly, it has been desired to discover a development inhibitor precursor which can properly control development, has a wide allowable range of processing temperature, and does not impair the pH controlling function of a cover sheet without reducing the image quality of color images after processing.